Skin is the largest organ of the human body and offers a direct route for therapeutics delivery. From a pharmaceutical point of view, the skin offers an outstanding route for therapeutic delivery with lots of advantages over other means of administration, including avoidance of first-pass metabolism, sustained and controlled delivery over a prolonged period of time, direct access to local target sites and improved patient compliance. Topical administration is the most challenging in non-invasive route of administration, and it is also the most promising alternative to injection for drug delivery to the diseased region. On the other hand, in the field of medical cosmetology, skin has always been the object of beautification and protection. Skin ageing gradually results in a series of skin problems, including dryness, gloom, wrinkling pigmentation and so on. In order to prevent skin aging or to improve these related symptoms, skin care products can be applied topically. The skin absorption of cosmetic active ingredients is the key for them to exert their specific functions of moisturing, whitening, anti-wrinkle, anti-aging and so on. Therefore, the research on improving the percutaneous absorption of drugs and cosmetic active ingredients is becoming increasingly widespread and the related technologies also have been gradually and widely applied to topical formulations.
However, skin is a biological barrier in essence against the invasion of external pathogens such as bacteria, viruses, or exogenous allergens. In general, only small lipophilic molecules (<500 Da) can penetrate into the skin at therapeutically adequate rates. Therefore, it is quite challenging to deliver various kinds of drugs or active substances with different physical and chemical properties, especially biological macromolecule drugs (polysaccharides, peptides, proteins, biological enzymes, nucleic acids, etc. having a molecular weight of at least 104 Da, 105 Da and even 106 Da) to the deep layers of the skin, which is also the key and focus of the researches in the field of skin drug delivery. So far, the technologies to increase percutaneous absorption of drugs can be generally classified into chemical methods and physical methods. Chemical methods include the use of penetration enhancers, nano-carrier systems (eg, liposomes, microemulsions, polymer nanoparticles, micells, quantum dots, gold nanoparticles), and so on. To marco biomolecules, the penetration enhancing effect from the chemical penetration enhancing techniques is limited. Further, the cytotoxicity and the metabolism in vivo of most chemical penetration enhancers and nanoparticle systems cannot be neglected. Thus, the application and industrialization of the chemical penetration enhancing techniques are restricted. On the other hand, physical penetration enhancing techniques include iontophoresis, ultrasound, electroporation, and microneedles and so on. Many of these techniques result in significant penetration enhancing effects. However, most of them are not portable and can only be applied within small areas of skin. Further, the design and manufacture cost is quite expensive. Thus, the application of physical penetration enhancing techniques is also limited. Among all those enhancement methods, microneedle is a physical penetration enhancing technology emerged in recent years. Microneedles can create plenty of microholes in skin, deep to the epidermis, even to the dermis, providing the most direct and effective way for the absorption of therapeutics. Theoretically, the therapeutics with any molecular size, polarity and other properties can penetrate into the pores formed by the microneedles. When the microneedles are applied to the skin, it is barely painful and the nerve tissue and micro-vessels are generally not injured and the punctured microholes within skin reduce and then close up in short time without causing skin damage.
The manufacturing method of microneedles is maturing day by day, but the choice of microneedle materials is always a problem. The mechanical strength of polymer microneedles is not enough to penetrate the dense stratum corneum; The application of monocrystalline silicon microneedle is limited due to its high brittleness, high manufacturing cost and controversial safety; the metal microneedle is much safer, however, the manufacture cost is quite expensive with the complicated process.
Sponge spicule is siliceous or calcareous fibrous substances as the skeleton in the marine sponge. There are uniaxial, triaxial, and multiaxial types of spicules. At present, most studies on the sponge spicules focus on their nanostructure, optical properties, growth mechanism, regulation, bionics, etc. The application sponge spicules as a skin penetration enhancer to increase percutaneous absorption of drugs and other therapeutics has not yet been fully exploited.